handshake

Performs SSL handshake, non blocking, but performs NEED_TASK on the same thread.
Hence, you should never call this method using your Acceptor thread, as you would slow down
your system significantly.
The return for this operation is 0 if the handshake is complete and a positive value if it is not complete.
In the event of a positive value coming back, reregister the selection key for the return values interestOps.